A model is presented for design of a thickener for solid-liquid separation where the flocs or aggregates within the solid-liquid suspension undergo an aggregate densification process due to the action of rakes. This aggregate densification facilitates suspension dewatering. The novel feature of the model is that it manages to couple together a hindered settling zone (higher up in the thickener, where the flocs are separated from one another, and the suspension cannot bear weight) and a gelled suspension zone (lower down in the thickener, where the flocs are packed together, and the suspension is able to bear weight). The model determines solids fraction profiles throughout the hindered settling zone and the gelled suspension zone, and also gives zone heights and residence times. Parametric investigations using the model are carried out for different suspension fluxes (which influence the solids fluxes and underflow solids fractions attained), and also for different specified amounts of and rates of aggregate densification.